Semiconductor fabrication processes are typically conducted with the substrate supported within a chamber under controlled conditions. For many processes, semiconductor substrates (e.g., silicon wafers) are heated inside the process chamber. For example, substrates can be heated by direct physical contact with a heated wafer holder and/or by radiation from a radiant heating source. “Susceptors,” for example, are wafer supports that absorb radiant heat and transmit absorbed heat to the substrate. Unless otherwise indicated, the terms “substrate” and “wafer” are used interchangeably herein.
In a typical process, a reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of reactant material on the wafer. Through sequential processing, multiple layers are made into integrated circuits. Other exemplary processes include sputter deposition, photolithography, dry etching, plasma processing, and high temperature annealing. Many of these processes require high temperatures and can be performed in the same or similar reaction chambers.
Various process parameters must be controlled carefully to ensure high quality deposited films. One critical parameter is the temperature of the wafer during processing. During CVD, for example, there is a characteristic temperature range within which the process gases react most efficiently for depositing a thin film onto the wafer. Temperature control is especially critical at temperatures below the mass transport regime, such as about 500° C. to 900° C. for silicon CVD using silane. In this kinetic regime, if the temperature is not uniform across the surface of the wafer, the deposited film thickness will be uneven.
In recent years, single-wafer processing of large diameter wafers has become more widely used for a variety of reasons, including the need for greater precision in process control than can be achieved with batch-processing. Typical wafers are made of silicon, most commonly with a diameter of about 150 mm (about 6 inches) or of about 200 mm (about 8 inches) and with a thickness of about 0.725 mm. Recently, larger silicon wafers with a diameter of about 300 mm (about 12 inches) and a thickness of about 0.775 mm have been utilized because they exploit the benefits of single-wafer processing even more efficiently. Even larger wafers are expected in the future. A typical single-wafer susceptor includes a pocket or recess within which the wafer rests during processing. In many cases, the recess is shaped to receive the wafer very closely.
There are a variety of quality control problems associated with handling of substrates. These problems include substrate slide, stick, and curl. These problems primarily occur during placement and subsequent removal of substrates in high temperature process chambers, particularly single-wafer chambers.
Substrate “slide” or “skate” occurs during drop off when a cushion of gas in the susceptor recess or pocket is unable to escape fast enough to allow the substrate to fall immediately onto the susceptor. The substrate floats momentarily above the susceptor as the gas slowly escapes, and it tends to drift off-center. Thus, the substrate may not rest in the center of the pocket as normally intended, and uneven heating of the substrate can result. Such drifting of the substrate to the edge of a susceptor pocket causes local thermal anomalies where the substrate is in contact with the pocket edge and results in poor thickness uniformity, poor resistivity uniformity, and crystallographic slip, depending on the nature of the layer being deposited. Non-uniformities in temperature can similarly cause non-uniformities in etching, annealing, doping, oxidation, nitridation, and other fabrication processes.
During substrate pick-up, “stick” occurs when the substrate clings to the underlying support because gas is slow to flow into the small space between the wafer and the surface of the substrate support pocket. This creates a vacuum effect between the substrate and the substrate support as the substrate is lifted. Stick can contribute to particle contamination due to scratching against the substrate support and, in extreme cases, can cause lifting of the substrate holder on the order of 1 to 2 mm.
Substrate “curl” is warping of the substrate caused by radial and axial temperature gradients in the substrate. Severe curl can cause the substrate to contact the bottom side of a Bernoulli wand when a cold substrate is initially dropped onto a hot substrate support. Curl can similarly affect interaction with other robotic substrate handling devices. In the case of a Bernoulli wand, the top side of the substrate can scratch the Bernoulli wand, causing particulate contamination on the substrate. This significantly reduces yield. The design and function of a Bernoulli wand are described in U.S. Pat. No. 5,997,588, the entire disclosure of which is hereby incorporated by reference herein.
FIGS. 1A and 1B show a wafer 1 supported upon a conventional susceptor 100, wherein the susceptor 100 has a gridded support surface G. Referring initially to FIG. 1A, a portion of the wafer 1, close to a peripheral edge 2 thereof, is shown on the grid G. An upper surface of the grid G includes a plurality of projections 3 separated from one another in two dimensions by a plurality of grid grooves 101. These projections 3 are recessed with respect to the upper surface of an annular shoulder 4 surrounding the grid. The top surface of the wafer 1 rises slightly above the top surface of the shoulder 4, which helps to maintain laminar gas flow over the wafer. An outer circumference 5 of the grid G is separated from an inner edge 6 of the shoulder 4 by an annular groove 7, which is generally semicircular in cross section. The depth of annular groove 7 into the susceptor 100 is about the same as the depth of the grid grooves. The diameter of the inner edge 6 of the shoulder 4 is slightly larger than the diameter of the wafer 1 to allow tolerance for positioning the wafer in the pocket. Similar gridded susceptors are commercially available from ASM America, Inc. of Phoenix, Ariz. for use in its Epsilon™ series of CVD reaction chambers.
In FIG. 1A, the wafer 1 is centered within the pocket such that there is uniform spacing between wafer edge 2 and shoulder edge 6 throughout the wafer periphery. FIG. 1A represents the ideal position of the wafer 1 with respect to the susceptor 100. However, as shown in FIG. 1B, upon initial placement the wafer 1 often tends to slide (upon drop-off) and/or jump (upon curl), and its outer edge 2 can contact or come in close proximity to the inner edge 6 of the shoulder 4. The shoulder 4 is thicker and thus generally cooler than the wafer 1 and the underlying grid G. As a result, the portion of the edge 2 of the wafer 1 in contact with the shoulder 4 tends to cool by conduction therebetween. This portion of the wafer edge 2 also tends to lose heat through radiation if it is very near to the shoulder edge 6, even if the wafer edge and the shoulder are not actually in contact.
Cooling at the wafer edge causes the temperature of the wafer to be non-uniform. Since thin film deposition rates (and many other fabrication processes) are often strongly temperature dependent, especially for CVD in the kinetic regime, film thickness and resistivity will be non-uniform across a wafer processed under conditions of temperature non-uniformity. Consequently, there is a need for an improved substrate support that facilitates substrate pick-up and drop-off while promoting temperature uniformity.